(a) Field of the invention
The present invention relates to a large-aperture quasi-telephoto lens system comprising a graded refractive index lens element (GRIN lens element), and more specifically a so-called axial type GRIN lens element having a refractive index varying in the direction along the optical axis (axial GRIN lens element).
(b) Description of the prior art
In recent years, when automatic focusing mechanisms are built in single lens reflex cameras and lens shutter cameras, zoom lens systems are incorporated in most cameras for permitting photographing at various field angles in simple procedures. However, since it is rather difficult to design a bright zoom lens system having a small F number, it is not so easy to correct aberrations sufficiently in a zoom lens system as compared with correction of aberrations in a lens system having a single focal length. In order to meet users'0 demand for a lens system which can provide bright images of high quality, it is therefore necessary to design a large-aperture lens system having a single focal length and providing high quality images.
If an attempt is made to design such a lens system by using spherical lens components only, the lens system will inevitably have a long total length and comprise a large number of lens components, thereby requiring high manufacturing cost and being low in portability. On the other hand, aspherical surfaces are used for obtaining a lens system comprising a small number of lens components and having favorably corrected aberrations. In case of a bright lens system having a small F number, however, each of the lens components will have a large diameter, thereby making it difficult to shape an aspherical lens component and increasing manufacturing cost of the lens system.
In order to correct the defect described above, it is practiced to use GRIN lens components having varying refractive indices. The GRIN lens components are classified into radial GRIN lens components whose refractive indices vary in the radial direction and axial GRIN lens components whose refractive indices vary in the direction along the optical axis. Out of these GRIN lens components, the radial GRIN lens components can hardly be used in lens systems for cameras since it is difficult to obtain radial GRIN lens components having large diameters by the manufacturing techniques currently available. On the other hand, it is possible to obtain axial GRIN lens components having large diameters which can provide the effect of the aspherical surface even when the lens components have spherical surfaces that are easily manufacturable.
As a conventional example of lens systems using the axial GRIN lens components, there is known the lens system disclosed by Japanese Unexamined Published Patent Application No. 149312/59.
However, the GRIN lens component used in the lens system mentioned above as the conventional example has no function to correct chromatic aberration. Further, this conventional example is a large-aperture standard lens system which is not a large-aperture quasi-telephoto lens system.